A DSL connection provides digital communication over existing twisted copper pair subscriber lines. The term DSL is a collective term to cover a number of variations on the DSL technology, including Asymmetric DSL (ADSL), Symmetric DSL (SDS), ADSL2+ and Very high data rate DSL (VDSL). A DSL connection comprises a copper subscriber line extending between two DSL modems. A first DSL modem is typically located at the customer's premises, and the second modem may be located at the local exchange (known as the ‘central office’ in US terminology), a street cabinet, or distribution point (sometimes known as ‘drop point’). Typically, the local exchange, street cabinet or distribution point includes a DSL Access Multiplier, DSL Access Multiplier (DSLAM) (a form of aggregation transceiver device) comprising several DSL modems (one for each subscriber line). The DSLAM (at the exchange, cabinet or distribution point) connects the first DSL modem at the customer's premises to the Core Network, typically over a faster optical fibre connection, and a Network Management System.
FIG. 1 illustrates a hierarchical relationship between the local exchange, street cabinet, distribution point and customer premises. The local exchange DSLAM is at level 1 of the hierarchy and connects a first set of customers' DSL modems to a Core Network. The street cabinet DSLAM is at level 2 of the hierarchy and connects a second set of customers' DSL modems to the Core Network through the exchange. The distribution point DSLAM is at level 3 of the hierarchy and connects a third set of customers' DSL modems to the Core Network through the exchange. All levels of the DSL hierarchy may transmit data over the same frequency domain.
It is well-known that transmissions on one subscriber line may cause interference on another subscriber line. This is known as ‘crosstalk’. Furthermore, it is common for the different sets of customers' DSL modems to be bundled together (for example, a subscriber line between one of the first set of customers' DSL modems and the local exchange may be bundled together with a subscriber line between one of the third set of customers' DSL modems and the distribution point, as shown in FIG. 1). However, subscriber lines between higher-tier DSLAMs and their customers' DSL modems tend to be significantly longer than subscriber lines for lower-tier DSLAMs, such that a signal on the longer subscriber line is significantly attenuated by the point at which it is bundled together with the subscriber lines for lower-tier DSLAMs. Thus, full power transmissions by lower-tier DSLAMs cause high levels of crosstalk on the higher-tier subscriber lines on the same frequency.
This problem has been addressed by a technique called the Access Network Frequency Plan, ANFP. The ANFP preserves resources for the exchange by defining the Power Spectral Density, PSD, of transmissions by lower-tier DSLAMs. For example, the PSD for the cabinet's DSLAM is shaped such that the power level at any frequency that is also being used by the exchange DSLAM is reduced. This decreases the chance of crosstalk on the DSL connections between the exchange and the customer. This technique may also be applied to the distribution point's DSLAM (such that the PSD is shaped according to the resources being used by the street cabinet and the exchange).
In order to provide customers with higher data rates, the location of the DSLAM is generally moving closer to the customer (i.e. from the exchange towards the distribution point). Thus, connections between the local exchange and the street cabinet and distribution points are being replaced with optical fibre, such that the length of the copper subscriber line (with its inherent data rate limitations) is reduced. There is therefore a trend in providing a greater number of DSLAMs in the street cabinets and distribution points.
The present inventors have identified a problem with the ANFP. Although the ANFP reduces the chance of crosstalk on the higher-tier DSL connections, the technique also reduces the capacity of the lower-tier DSLAMs when there is an overlap in the frequencies used by both the lower and higher tier DSLAMs. As DSLAMs migrate closer to the customer, the ANFP technique will become increasingly more inefficient in terms of the total capacity of the network.
Furthermore, the prior art techniques involve the Network Management System determining the resource allocations for subscriber lines offline. This is a manual and time-consuming process for the Network Operator. Once the resources have been allocated, the Network Management System sends an allocation message to the DSLAM, which then implements the resource allocation to the line on the next retrain. These retrains are particularly frustrating for the end-user as they result in several minutes of loss of service. Accordingly, the Network Operator must either wait for the line to retrain, or force a retrain on the line at an appropriate time (e.g. during the middle of the night). However, either way, there is a significant delay before the resource allocations for all lines in the network have been implemented.
It is therefore desirable to alleviate some or all of the above problems.